The present invention generally relates to processes for the production of tubular packaging sheaths made from cellulose hydrate with a uniformly uninterrupted film coating of natural or synthetic polymer on the inside, the coating being virtually impermeable to water and water vapor, and to an apparatus for carrying out the processes. More particularly, the invention relates to processes for the production of a tubular packaging sheath for pasty foodstuffs, especially as a synthetic sausage casing for sausage intended to be boiled or cooked and to an apparatus for carrying out the processes.
British Patent Specification No. 1,201,830 describes a process for the internal coating of tubular packaging sheaths made from cellulose hydrate with an aqueous polymeric dispersion by continuously passing the tubing horizontally through the nip of a vertical pair of nip rolls and thereafter deflecting the tubing to a vertical direction by means of a deflecting roller. The cavity of the portion of tubing issuing from the nip of the pair of nip rolls contains a certain amount of aqueous polymeric dispersion as a coating liquid. The vertically running tubing is exposed to heat above the level of the quantity of coating liquid enclosed in the cavity of the tubing.
In a process for the internal coating of cellulose hydrate tubing with an aqueous plastic solution disclosed in U.S. Pat. No. 2,901,358, the tubing to be coated is first guided horizontally in a flattened form and then partially wrapped around the circumference of one roll of a horizontal pair of nip rolls, passed through the nip of this pair and thereafter guided vertically upwards. A certain amount of coating liquid is enclosed in the tubing cavity above the nip of the pair of rolls. The exposure to heat, in order to dry the layer on the inside of the tubing, takes place after the tubing is pressed flat in the nip of a pair of nip rolls.
German Offenlegungsschrift No. 2,801,038 discloses a process for the internal coating of tubing wherein tubing made of plastic or of a dense fabric is continuously passed through the metering nip of a vertical pair of nip rolls and then is passed in an upwardly-inclined direction. Before entering the nip of the pair of rolls, the tubing first rests, in flattened form, partially against the circumference of the lower roll of the pair of rolls, and after issuing from the metering nip it rests, in flattened form, partially against the upper roll. After the flattened tubing has been lifted off the surface of the upper roll of the pair of rolls, it proceeds in an upwardly-inclined direction. The coating liquid contained in the portion of the tubing, is upstream of the pair of nip rolls.
The known processes for the internal coating of cellulose hydrate tubing with aqueous polymeric dispersions all have the disadvantage that the cellulose hydrate tubing, which, because of the high water content of the polymeric dispersion used, is heavily moistened with water, tends on subsequent drying to undergo an undesirable change of dimension due to shrinkage. They further all have the disadvantage that because of the use of aqueous polymeric dispersions having a relatively low content of dispersed polymer, the amount of water which has to be expelled in order to form the film coating on the inside of the tubing by exposing the tubing to heat, and transported outwards through the tubing wall by diffusion, is large in relation to the amount of the dispersed polymer component of the dispersion. The known processes accordingly are disadvantageous in energy terms.
In the text which follows, a horizontal pair of rolls is to be understood as a pair of rolls in which a common substantially horizontal plane can pass through the parallel axes of its rolls. A vertical pair of rolls is to be understood, by definition, as a pair of rolls in which the parallel axes of its rolls extend in a common substantially vertical plane.